Process to Prepare a Lubricating Base Oil

ABSTRACT

Process to prepare a base oil having an paraffin content of between 75 and 95 wt % by subjecting a mixture of a Fischer-Tropsch derived feed and a gas field condensate derived feed to a catalytic pour point reducing treatment.

The invention is directed to a process to prepare a base oil having anparaffin content of between 75 and 95 wt %.

WO-A-0246333 describes a process to prepare two viscosity grades of baseoil, by solvent dewaxing a fraction having a T95% point above 621° C.and catalytically dewaxing a fraction having a T95% point of below 621°C. The two fractions are Fischer-Tropsch derived fractions. Optionallythe heavier or the lower boiling fraction may also be a slack wax, adistillate from crude oil or deasphalted residual stocks from crude oil.

NL-C-1015035 describes a process to prepare a base oil from aFischer-Tropsch derived feed by performing a hydroisomerisation step.The effluent of the hydroisomerisation step is distilled and a residueboiling above 380° C. is obtained. This residue is subjected to acatalytic dewaxing treatment using a catalyst containing platinum andferrierite.

U.S. Pat. No. 6,294,077 describes a catalytic dewaxing treatment whereina catalyst is used consisting of ZSM-5 and platinum.

U.S. Pat. No. 6,025,305 discloses a process wherein a Fischer-Tropschwax feed is first hydroisomerised. The effluent of thehydroisomerisation is then separated into fuels and lubricants. No pourpoint reducing treatment is disclosed in this publication.

US-A-2002/0146358 describes a process for hydroisomerisation of aFischer-Tropsch derived wax feed. The effluent of the hydroisomerisationstep is distilled and a bottoms fraction comprising compounds having 20or more carbon atoms is obtained. This bottoms fraction may be subjectedto a catalytic dewaxing treatment.

WO-A-0157166 describes the use of a highly paraffinic base oil asobtained from a Fischer-Tropsch wax in a motor engine lubricantformulation. The examples illustrate that such formulations will alsoconsist of an ester, which according to the description of the patent isadded to confer additional desired characteristics, such as additivesolvency.

The use of ester co-base fluids in lubricant formulations as illustratedin WO-A-0157166 is not desired because such ester co-base fluids are notwidely available and thus expensive. Additive solvency may be improvedby using a paraffinic base stock, which contains less paraffins. Suchbase oils may be prepared by hydroisomerisation of petroleum derivedwaxes, followed by a solvent or catalytic dewaxing step. A disadvantageof such a process is that the starting petroleum derived waxes, such asfor example slack wax, are not easily obtainable. Furthermore, suchwaxes may not always have the desired high paraffin content needed tomake the desired base oils as per this invention.

The object of the present invention is to provide a process wherein abase oil with a paraffin content of between 75 and 95 wt % is obtainedwhich does not have the disadvantages of the prior art processes.

This object is achieved by the following process. Process to prepare abase oil having a paraffin content of between 75 and 95 wt %, bysubjecting a mixture of a Fischer-Tropsch derived feed and a gas fieldcondensate derived feed to a catalytic pour point reducing treatment.

Applicants found that by using a mixture of a relatively small amount ofa gas field condensate derived feed with a Fischer-Tropsch derived feedbefore performing a catalytic pour point reducing treatment a base oilmay be obtained having the desired properties. A further advantage isthat the gas field condensate is obtained when natural gas fields aredeveloped. These condensates are the liquid fraction associated with themajor compound methane. Fischer-Tropsch processes typically derivedtheir feed from natural gas. Thus it is advantageous to use the gasfield condensate in a process to prepare base oils in a Fischer-Tropschfacility based on natural gas because it simplifies logistics.

Preferably from the gas field condensate a fraction is isolated by meansof for example distillation, which boils for more than 80 wt % above340° C., more preferably for more than 80 wt % above 370° C. and evenmore preferably for more than 80 wt % above 390° C. These fractions maybe recovered directly from the gas field condensate as it is separatedfrom the natural gas or may be obtained in dedicated gas fieldcondensate refineries which refineries upgrade the gas field condensateas obtained in a multitude of natural gas wells.

The gas field condensate normally will contain sulphur. If the sulphurcontent is too high the condensate is preferably treated to reduce thesulphur content to levels below 20 ppm, preferably below 10 ppm in thefraction to be used in the present process. The sulphur level in thecombined mixture used in the process of the present invention ispreferably below 10 ppm. Treating is advantageously performed by meansof hydrotreating or by means of sorption processes. Hydrotreating is awell-known process and may be performed with a supported Co/Mo catalyst.In such hydrotreating the level of aromatics and nitrogen is alsopreferably reduced. The fraction comprising the gas field condensatepreferably has an aromatic compound content of between 0 and 20 wt % anda naphthenic compound content of preferably between 15 and 90 wt %. Thecomposition is analysed using the following technique.

An example of a possible sorbent processes for desulfurizing the gasfield condensate is a process wherein the gas field condensate iscontacted with a sorbent comprising zinc oxide under conditionssufficient to remove at least a portion of the sulfur from the fluidstream and provide a sulfurized sorbent comprising zinc sulfide. Thesulfurized sorbent is thereafter contacted with an oxygen-containingregeneration stream under conditions sufficient to convert at least aportion of the zinc sulfide to zinc oxide, thereby providing aregenerated sorbent. The regenerated sorbent can then be contacted witha reducing stream to provide an activated sorbent. Thereafter, theactivated sorbent can, once again, be contacted with the gas fieldcondensate. Examples of such processes are described in WO-A-0016895,U.S. Pat. No. 4,990,318, U.S. Pat. No. 5,077,261, U.S. Pat. No.5,102,854, U.S. Pat. No. 5,108,975, U.S. Pat. No. 5,130,288, U.S. Pat.No. 5,174,919, U.S. Pat. No. 5,177,050, U.S. Pat. No. 5,219,542, U.S.Pat. No. 5,244,641, U.S. Pat. No. 5,248,481, U.S. Pat. No. 5,281,445 andU.S. Pat. No. 6,544,410.

The cyclo-paraffin (naphthenic compounds) content in this mixture ofcyclo-, normal and iso-paraffins is measured by the following method.Any other method resulting in the same results may also be used. Thebase oil sample is first separated into a polar (aromatic) phase and anon-polar (saturates) phase by making use of a high performance liquidchromatography (HPLC) method IP368/01, wherein as mobile phase pentaneis used instead of hexane as the method states. The saturates andaromatic fractions are then quantatively analyzed using a Finnigan MAT90mass spectrometer equipped with a Field desorption/Field Ionisation(FD/FI) interface, wherein FI (a “soft” ionisation technique) is usedfor the quantitative determination of hydrocarbon types in terms ofcarbon number and hydrogen deficiency of this particular base oilfraction. The instrument conditions to achieve such a soft ionizationtechnique are a source temperature of 30° C., an extraction voltage of 5kV, an emitter current of 5 mA and a probe temperature ramp of 40° C. to400° C. (20° C./min)

The type classification of compounds in mass spectrometry is determinedby the characteristic ions formed and is normally classified by “znumber”. This is given by the general formula for all hydrocarbonspecies: CnH₂n+z. Because the saturates phase is analysed separatelyfrom the aromatic phase it is possible to determine the content of thedifferent (cyclo)-paraffins having the same stoichiometry. The resultsof the mass spectrometer are processed using commercial software (poly32; available from Sierra Analytics LLC, 3453 Dragoo Park Drive,Modesto, Calif. GA95350 USA) to determine the relative proportions ofeach hydrocarbon type and the average molecular weight andpolydispersity of the saturates and aromatics fractions.

The Fischer-Tropsch derived feed preferably is a hydroisomerizedFischer-Tropsch wax. Such a feed may be obtained by well-knownprocesses, for example the so-called commercial Sasol process, the ShellMiddle Distillate Process or by the non-commercial Exxon process. Theseand other processes are for example described in more detail inEP-A-776959, EP-A-668342, U.S. Pat. No. 4,943,672, U.S. Pat. No.5,059,299, WO-A-9934917 and WO-A-9920720. The process will generallycomprise a Fischer-Tropsch synthesis and a hydroisomerisation step asdescribed in these publications.

In another preferred embodiment of the present invention the, optionallypartly desulphirized, gas field condensate comprising at least thefraction boiling in the base oil range as described above is subjectedto the hydroisomerisation step together with the Fischer-Tropsch wax.This is advantageous because then sulphided non-noble catalyst asdescribed below can be used in this process step, while at the same timeimproving the yield to hydrocarbon products from the natural gas well.The fact that the products will contain some sulphur and othernon-paraffinic compounds does not have to be a disadvantage if onerealises that such products will be blended with mineral oil productscomprising sulphur and non-paraffinic compounds anyway.

A preferred process to prepare the hydroisomerised Fischer-Tropsch feedor the blend of a hydroisomerised Fischer-Tropsch feed and the gas fieldcondensate derived feed for use as feed in the present process willcomprise the following steps:

-   (a) hydrocracking/hydroisomerisating a Fischer-Tropsch product or    the mixed feed,-   (b) separating by means of distillation the product of step (a) into    one or more gas oil fractions and a higher boiling Fischer-Tropsch    derived feed according to this invention.

Optionally the hydroisomerisation step on the mixed feed can beperformed at such a high conversion/isomerisation rate that a separatedewaxing step to obtain the desired base oil grade having the desiredpour point can be omitted. In such a situation it is obvious that saidhydroisomerisation step is actually the dewaxing step according to thepresent invention.

Preferably the Fischer-Tropsch product used as feed in step (a) is aproduct wherein the weight ratio of compounds having at least 60 or morecarbon atoms and compounds having at least 30 carbon atoms in theFischer-Tropsch product is at least 0.2 and wherein at least 30 wt % ofcompounds in the Fischer-Tropsch product have at least 30 carbon atoms.

Applicants found that by performing thehydro-cracking/hydroisomerisation step with the relatively heavyfeedstock a higher yield of gas oils as calculated on the feed to step(a) can be obtained. A further advantage is that both fuels, for examplegas oil, and the Fischer-Tropsch derived feed are prepared in onehydrocracking/hydroisomerisation process step. In a preferred embodimentof the present invention a fraction boiling above the Fischer-Tropschderived feed is isolated in step (b) and recycled to step (a).

A further advantage is that by performing step (a) on the relativelyheavy feed a Fischer-Tropsch derived feed is prepared which already hasa certain content of cyclo-paraffins.

The relatively heavy Fischer-Tropsch product used in step (a) has morepreferably at least 50 wt %, and even more preferably at least 55 wt %of compounds having at least 30 carbon atoms. Furthermore the weightratio of compounds having at least 60 or more carbon atoms and compoundshaving at least 30 carbon atoms of the Fischer-Tropsch product is morepreferably at least 0.4 and even more preferably at least 0.50.Preferably the Fischer-Tropsch product comprises a C₂₀ ⁺ fraction havingan ASF-alpha value (Anderson-Schulz-Flory chain growth factor) of atleast 0.925, preferably at least 0.935, more preferably at least 0.945,even more preferably at least 0.955.

The initial boiling point of the Fischer-Tropsch product used in step(a) may range up to 400° C., but is preferably below 200° C. Preferablyany compounds having 4 or less carbon atoms and any compounds having aboiling point in that range are separated from a Fischer-Tropschsynthesis product before the Fischer-Tropsch synthesis product is usedin step (a). The Fischer-Tropsch product as described in detail above isa Fischer-Tropsch product, which has not been subjected to ahydroconversion step as defined according to the present invention. Thecontent of non-branched compounds in the Fischer-Tropsch product willtherefore be above 80 wt %. In addition to the Fischer-Tropsch productand the optional gas field condensate also other fractions may beadditionally processed in step (a). Possible other fractions maysuitably be the optional higher boiling fraction obtained in step (b) orpart of said fraction and/or off-spec base oil fractions as obtained inthe pour point reducing treatment of the process of the presentinvention.

Such a Fischer-Tropsch product can be obtained by any process, whichyields a relatively heavy Fischer-Tropsch product as described above.Not all Fischer-Tropsch processes yield such a heavy product. An exampleof a suitable Fischer-Tropsch process is described in WO-A-9934917 andin AU-A-698392. These processes may yields a Fischer-Tropsch product asdescribed above.

The Fischer-Tropsch product will contain no or very little sulphur andnitrogen containing compounds. This is typical for a product derivedfrom a Fischer-Tropsch reaction, which uses synthesis gas containingalmost no impurities. Sulphur and nitrogen levels will generally bebelow the detection limits, which are currently 2 ppm for sulphur and 1ppm for nitrogen respectively.

The Fischer-Tropsch product may optionally be subjected to a mildhydrotreatment step in order to remove any oxygenates and saturate anyolefinic compounds present in the reaction product of theFischer-Tropsch reaction. Such a hydrotreatment is described inEP-B-668342. The mildness of the hydrotreating step is preferablyexpressed in that the degree of conversion in this step is less than 20wt % and more preferably less than 10 wt %. The conversion is heredefined as the weight percentage of the feed boiling above 370° C.,which reacts to a fraction boiling below 370° C. After such a mildhydrotreatment lower boiling compounds, having four or less carbon atomsand other compounds boiling in that range, will preferably be removedfrom the effluent before it is used in step (a).

The hydrocracking/hydroisomerisation reaction of step (a) is preferablyperformed in the presence of hydrogen and a catalyst, which catalyst canbe chosen from those known to one skilled in the art as being suitablefor this reaction of which some will be described in more detail below.The catalyst may in principle be any catalyst known in the art to besuitable for isomerising paraffinic molecules. In general, suitablehydroconversion catalysts are those comprising a hydrogenation componentsupported on a refractory oxide carrier, such as amorphoussilica-alumina, alumina, fluorided alumina, molecular sieves (zeolites)or mixtures of two or more of these. One type of preferred catalysts tobe applied in the hydroconversion step in accordance with the presentinvention are hydroconversion catalysts comprising platinum and/orpalladium as the hydrogenation component. A very much preferredhydroconversion catalyst comprises platinum and palladium supported onan amorphous silica-alumina (ASA) carrier. The platinum and/or palladiumis suitably present in an amount of from 0.1 to 5.0% by weight, moresuitably from 0.2 to 2.0% by weight, calculated as element and based ontotal weight of catalyst. If both present, the weight ratio of platinumto palladium (calculated as element) may vary within wide limits, butsuitably is in the range of from 0.05 to 10, more suitably 0.1 to 5.Examples of suitable noble metal on ASA catalysts are, for instance,disclosed in WO-A-9410264 and EP-A-0582347. Other suitable noblemetal-based catalysts, such as platinum on a fluorided alumina carrier,are disclosed in e.g. U.S. Pat. No. 5,059,299 and WO-A-9220759.

A second type of suitable hydroconversion catalysts are those comprisingat least one Group VIB metal, preferably tungsten and/or molybdenum, andat least one non-noble Group VIII metal, preferably nickel and/orcobalt, as the hydrogenation component. Usually both metals are presentas oxides, sulphides or a combination thereof. The Group VIB metal issuitably present in an amount of from 1 to 35% by weight, more suitablyfrom 5 to 30% by weight, calculated as element and based on total weightof catalyst. The non-noble Group VIII metal is suitably present in anamount of from 1 to 25% wt, preferably 2 to 15% wt, calculated aselement and based on total weight of catalyst. A hydroconversioncatalyst of this type which has been found particularly suitable is acatalyst comprising nickel and tungsten supported on fluorided alumina.

A preferred catalyst which can be used in a non-sulphided form comprisesa non-noble Group VIII metal, e.g., iron, nickel, in conjunction with aGroup IB metal, e.g., copper, supported on an acidic support. Thecatalyst has a surface area in the range of 200-500 m²/gm, preferably0.35 to 0.80 ml/gm, as determined by water adsorption, and a bulkdensity of about 0.5-1.0 g/ml. The catalyst support is preferably anamorphous silica-alumina where the alumina is present in amounts of lessthan about 30 wt %, preferably 5-30 wt %, more preferably 10-20 wt %.Also, the support may contain small amounts , e.g., 20-30 wt %, of abinder, e.g., alumina, silica, Group IVA metal oxides, and various typesof clays, magnesia, etc., preferably alumina.

The preparation of amorphous silica-alumina microspheres has beendescribed in Ryland, Lloyd B., Tamele, M. W., and Wilson, J. N.,Cracking Catalysts, Catalysis: volume VII, Ed. Paul H. Emmett, ReinholdPublishing Corporation, New York, 1960, pp. 5-9.

The catalyst is prepared by co-impregnating the metals from solutionsonto the support, drying at 100-150° C., and calcining in air at200-550° C. The Group VIII metal is present in amounts of about 15 wt %or less, preferably 1-12 wt %, while the Group IB metal is usuallypresent in lesser amounts, e.g., 1:2 to about 1:20 weight ratiorespecting the Group VIII metal.

A typical catalyst is shown below: Ni, wt % 2.5-3.5 Cu, wt % 0.25-0.35Al₂O₃—SiO2 wt % 65-75 Al₂O₃ (binder) wt % 25-30 Surface Area 290-325m²/g Pore Volume (Hg) 0.35-0.45 ml/g Bulk Density 0.58-0.68 g/ml

Another class of suitable hydroconversion catalysts are those based onzeolitic materials, suitably comprising at least one Group VIII metalcomponent, preferably Pt and/or Pd, as the hydrogenation component.Suitable zeolitic materials, then, include Zeolite beta, Zeolite Y,Ultra Stable Y, ZSM-5, ZSM-12, ZSM-22, ZSM-23, ZSM-35, SSZ-32,ferrierite, mordenite and silica-alumino-phosphates, such as SAPO-11 andSAPO-31. Examples of suitable hydroisomerisation catalysts and processesare, for instance, described in WO-A-9201657, WO-A-0107538 orEP-A-1029029 and US-A-20040065581.

A process wherein step (b) can be omitted is for example described inUS-A-20040065581 or in EP-A-1029029. These publications describe theconversion of a narrow boiling Fischer-Tropsch derived wax to a base oilby contacting the feed with a platinum/zeolite beta followed by directlycontacting the effluent with a platinum/ZSM-48 or platinum/ZSM-23dewaxing catalyst. In such a line-up the petroleum derived feed may beadvantageously added prior to contacting with the platinum/ZSM-48 orplatinum/ZSM-23 catalyst according to the present invention.

In step (a) the feed is contacted with hydrogen in the presence of thecatalyst at elevated temperature and pressure. The temperaturestypically will be in the range of from 175 to 425° C., preferably higherthan 250° C. and more preferably from 280 to 400° C. The hydrogenpartial pressure will typically be in the range of from 10 to 250 barand preferably between 20 and 100 bar. The hydrocarbon feed may beprovided at a weight hourly space velocity of from 0.1 to 5 kg/l/hr(mass feed/volume catalyst bed/time), preferably higher than 0.5 kg/l/hrand more preferably lower than 2 kg/l/hr. Hydrogen may be provided at aratio of hydrogen to hydrocarbon feed from 100 to 5000 Nl/kg andpreferably from 250 to 2500 Nl/kg.

The conversion in step (a) as defined as the weight percentage of thefeed boiling above 370° C. which reacts per pass to a fraction boilingbelow 370° C., is at least 20 wt %, preferably at least 25 wt %, butpreferably not more than 90 wt %. The feed as used above in thedefinition is the total hydrocarbon feed fed to step (a), thus also anyoptional recycle of the higher boiling fraction as obtained in step (b).

In step (b) the product of step (a) is separated into one or more gasoil fractions and a Fischer-Tropsch derived feed having preferably a T10wt % boiling point of between 200 and 450° C.

The fraction of the gas field condensate in the mixture is preferablyhigher than 5 wt %, more preferably higher than 10 wt % and preferablylower than 50 wt % and more preferably below 30 wt % and even morepreferably below 25 wt %. The actual content of the fraction comprisingthe gas field condensate in the mixture will of course depend on theparaffin content of said feed. The mixture will preferably contain lessthan 50 ppm sulphur and/or less that 10 ppm nitrogen.

With the catalytic pour point reducing treatment is understood everyprocess wherein the pour point of the base oil is reduced by more than10° C., preferably more than 20° C., more preferably more than 25° C.

The catalytic dewaxing or pour point reducing process can be performedby any process wherein in the presence of a catalyst and hydrogen thepour point of the mixture is reduced as specified above. Suitabledewaxing catalysts are heterogeneous catalysts comprising a molecularsieve and optionally in combination with a metal having a hydrogenationfunction, such as the Group VIII metals. Molecular sieves, and moresuitably intermediate pore size zeolites, have shown a good catalyticability to reduce the pour point of the distillate base oil precursorfraction under catalytic dewaxing conditions. Preferably theintermediate pore size zeolites have a pore diameter of between 0.35 and0.8 nm. Suitable intermediate pore size zeolites are zeolite beta,mordenite, ZSM-5, ZSM-12, ZSM-22, ZSM-23, SSZ-32, ZSM-35 and ZSM-48 orcombinations of said zeolites. Another preferred group of molecularsieves are the silica-aluminaphosphate (SAPO) materials of which SAPO-11is most preferred as for example described in U.S. Pat. No. 4,859,311.ZSM-5 may optionally be used in its HZSM-5 form in the absence of anyGroup VIII metal. The other molecular sieves are preferably used incombination with an added Group VIII metal or mixtures of said metals.Suitable Group VIII metals are nickel, cobalt, platinum and palladium.Examples of possible combinations are Ni/ZSM-5, Pt/ZSM-23, Pd/ZSM-23,Pt/ZSM-48 and Pt/SAPO-11 or stacks of Pt/zeolite beta and Pt/ZSM-22;Pt/zeolite beta and Pt/ZSM-23; and Pt/zeolite beta and Pt/ZSM-48.Further details and examples of suitable molecular sieves and dewaxingconditions are for example described in WO-A-9718278, U.S. Pat. No.5,053,373, U.S. Pat. No. 5,252,527 and U.S. Pat. No. 4,574,043.

The dewaxing catalyst suitably also comprises a binder. The binder canbe a synthetic or naturally occurring (inorganic) substance, for exampleclay, silica and/or metal oxides. Natural occurring clays are forexample of the montmorillonite and kaolin families. The binder ispreferably a porous binder material, for example a refractory oxide ofwhich examples are: alumina, silica-alumina, silica-magnesia,silica-zirconia, silica-thoria, silica-beryllia, silica-titania as wellas ternary compositions for example silica-alumina-thoria,silica-alumina-zirconia, silica-alumina-magnesia andsilica-magnesia-zirconia. More preferably a low acidity refractory oxidebinder material which is essentially free of alumina is used. Examplesof these binder materials are silica, zirconia, titanium dioxide,germanium dioxide, boria and mixtures of two or more of these of whichexamples are listed above. The most preferred binder is silica.

A preferred class of dewaxing catalysts comprise intermediate zeolitecrystallites as described above and a low acidity refractory oxidebinder material which is essentially free of alumina as described above,wherein the surface of the aluminosilicate zeolite crystallites has beenmodified by subjecting the aluminosilicate zeolite crystallites to asurface dealumination treatment. A preferred dealumination treatment isby contacting an extrudate of the binder and the zeolite with an aqueoussolution of a fluorosilicate salt as described in for example U.S. Pat.No. 5,157,191 or WO-A-2000029511. Examples of suitable dewaxingcatalysts as described above are silica bound and dealuminated Pt/ZSM-5,silica bound and dealuminated Pt/ZSM-23, silica bound and dealuminatedPt/ZSM-12, silica bound and dealuminated Pt/ZSM-22 as for exampledescribed in WO-A-200029511 and EP-B-832171.

Catalytic dewaxing conditions are known in the art and typically involveoperating temperatures in the range of from 200 to 500° C., suitablyfrom 250 to 400° C., hydrogen partial pressures in the range of from 10to 200 bar, preferably from 15 to 100 bar, weight hourly spacevelocities (WHSV) in the range of from 0.1 to 10 kg of oil per litre ofcatalyst per hour (kg/l/hr), suitably from 0.2 to 5 kg/l/hr, moresuitably from 0.5 to 3 kg/l/hr and hydrogen to oil ratios in the rangeof from 100 to 2,000 litres of hydrogen per litre of oil. By varying thetemperature between 315 and 375° C. at a hydrogen partial pressure ofbetween 15-100 bars, in the catalytic dewaxing step it is possible toprepare base oils having different pour points varying from suitablylower than −60 to −10° C.

Optionally a noble metal guard bed may be positioned just upstream thedewaxing step, for example as a separate catalyst bed in the dewaxingreactor. Such a guard bed is advantageous to remove any remainingsulphur and especially nitrogen compounds present in the feed to thedewaxing process of the present invention.

After performing the pour point reducing treatment lower boilingcompounds formed during said treatment are suitably removed, preferablyby means of distillation, optionally in combination with an initialflashing step.

The effluent of the pour point reducing treatment may suitably besubjected to a hydrogenation treatment. Hydrogenation may be performedon the entire effluent or on specific base oil grades after the abovedescribed fractionation. This may be required in order to reduce thecontent of aromatic compounds in the reduced pour point product topreferably values of below 1 wt %. Such a hydrogenation is also referredto as a hydrofinishing step. This step is suitably carried out at atemperature between 180 and 380° C., a total pressure of between 10 to250 bar and preferably above 100 bar and more preferably between 120 and250 bar. The WHSV (Weight hourly space velocity) ranges from 0.3 to 2 kgof oil per litre of catalyst per hour (kg/l.h). Preferably ahydrogenation is performed in the same reactor as the catalytic dewaxingreactor. In such a reactor the beds of dewaxing catalyst andhydrogenation catalyst will be placed in a stacked bed on top of eachother.

The hydrogenation catalyst is suitably a supported catalyst comprising adispersed Group VIII metal. Possible Group VIII metals are cobalt,nickel, palladium and platinum. Cobalt and nickel containing catalystsmay also comprise a Group VIB metal, suitably molybdenum and tungsten.Suitable carrier or support materials are low acidity amorphousrefractory oxides. Examples of suitable amorphous refractory oxidesinclude inorganic oxides, such as alumina, silica, titania, zirconia,boria, silica-alumina, fluorided alumina, fluorided silica-alumina andmixtures of two or more of these.

Examples of suitable hydrogenation catalysts are nickel-molybdenumcontaining catalyst such as KF-847 and KF-8010 (AKZO Nobel) M-8-24 andM-8-25 (BASF), and C-424, DN-190, HDS-3 and HDS-4 (Criterion);nickel-tungsten containing catalysts such as NI-4342 and NI-4352(Engelhard) and C-454 (Criterion); cobalt-molybdenum containingcatalysts such as KF-330 (AKZO-Nobel), HDS-22 (Criterion) and HPC-601(Engelhard). Preferably platinum containing and more preferably platinumand palladium containing catalysts are used. Preferred supports forthese palladium and/or platinum containing catalysts are amorphoussilica-alumina. Examples of suitable silica-alumina carriers aredisclosed in WO-A-9410263. A preferred catalyst comprises an alloy ofpalladium and platinum preferably supported on an amorphoussilica-alumina carrier of which the commercially available catalystC-624 of Criterion Catalyst Company (Houston, Tex.) is an example.

After performing the catalytic pour point reducing treatment or afterthe optional hydrofinishing step hydrogen is suitably separated from thedewaxed/hydrofinished effluent, contacted with a means to removehydrogen sulphide and recycled to said catalytic pour point reducingtreatment. Suitably the hydrogen is contacted with a heterogeneousadsorbent selective for removing hydrogen sulphide. Preferably hydrogenis contacted with zinc oxide to remove hydrogensulphide.

From the effluent of the pour point reducing treatment and the optionalhydrogenation treatment one or more base oil grades may be isolated bymeans of fractionation. Base oil products having kinematic viscosity at100° C. of between 2 and 10 cSt, having a volatility of between 8 and11% (according to CEC L40 T87) and a pour point of between −20 and −60°C. (according to ASTM D 97) may advantageously be obtained.

The content of paraffins is more preferably less than 90 wt % and morepreferably higher than 80 wt %.

The above-described base oil can suitably find use as base oil for anAutomatic Transmission Fluids (ATF), motor engine oils, electrical oilsor transformer oils and refrigerator oils. Lubricant formulations suchas motor engine oils of the 0W-x and 5W-x specification according to theSAE J-300 viscosity classification, wherein x is 20, 30, 40, 50 or 60may be advantageously made using this base oil.

It has been found that lubricant formulations can be prepared with thebase oils obtainable by the process of the current invention without theneed to add high contents of additional ester or aromatic co-base oils.Preferably less than 15 wt % and more preferably less than 10 wt % ofsuch ester or aromatic co-base oil is present in such formulations.

1. A process to prepare a base oil having a paraffin content of between75 and 95 wt % comprising subjecting a mixture of a Fischer-Tropschderived feed and a gas field condensate derived feed to a catalytic pourpoint reducing treatment, wherein the gas field condensate has anaromatic content of between 0 and 20 wt % and a naphthenic compoundcontent of between 15 and 90 wt %.
 2. The process according to claim 1,wherein the content of sulphur in the mixture subjected to the pourpoint reducing treatment is below 50 ppm and the content of nitrogen inthe mixture subjected to the pour point reducing treatment is below 10ppm.
 3. The process according to claim 1, wherein the fraction of gasfield condensate in the mixture is higher than 5 wt % and lower than 50wt %.
 4. The process according to claim 1, wherein the base oil ishydrogenated after performing the pour point reducing treatment suchthat the content of aromatics is below 1 wt %.
 5. The process accordingto claim 1, wherein the catalytic pour point reducing treatment is acatalytic dewaxing process performed in the presence of a catalystcomprising a Group VIII metal and an intermediate pore size zeolitehaving pore diameter between 0.35 and 0.8 nm, and a binder.
 6. Theprocess according to claim 1, wherein after performing the catalyticpour point reducing treatment hydrogen is separated from the dewaxedeffluent, contacted with a heterogeneous adsorbent selective forremoving hydrogen sulphide and recycled to said catalytic pour pointreducing treatment.
 7. The process according to claim 6, wherein theheterogeneous adsorbent is zinc oxide.
 8. The process according to claim1, wherein the Fischer-Tropsch derived feed is obtained byhydroisomerisation of a Fischer-Tropsch product.
 9. The processaccording to claim 1, wherein the feed to the catalytic dewaxing step isobtained by hydroisomerisation of a mixture of a Fischer-Tropsch productand a gas field condensate.
 10. The process according to claim 8,wherein the hydroisomerised Fischer-Tropsch feed is obtained by means ofthe following steps: (a) hydrocracking/hydroisomerisating aFischer-Tropsch product, (b) separating by means of distillation theproduct of step (a) into one or more gas oil fractions and a higherboiling Fischer-Tropsch derived feed.
 11. The process according to claim1, wherein the Fischer-Tropsch product used as feed in step (a) is aproduct wherein the weight ratio of compounds having at least 60 or morecarbon atoms and compounds having at least 30 carbon atoms in theFischer-Tropsch product is at least 0.4 and wherein at least 30 wt % ofcompounds in the Fischer-Tropsch product have at least 30 carbon atoms.